Numerous methods for the detection of misfires in internal combustion engines are already known; see, for example, EP 0 708 234, EP 0 716 298 and U.S. Pat. No. 5,056,360. Said methods make use of the physical effect that a cylinder in which a misfire occurs exhibits a smaller acceleration value than adjacent cylinders. In the methods known in the prior art, this physical effect is made use of in such a way that a parameter dependent on the acceleration of the internal combustion engine, such as, for example, an acceleration index or what is known as a cylinder segment time is continually determined by means of a monitoring and analysis method while the internal combustion engine is running. Said parameter is then compared with a threshold value. The threshold value is defined as a function of the operating point of the internal combustion engine (e.g. as a function of the speed and load), and it is generally stored once during the calibration of the internal combustion engine in the operation control device of the internal combustion engine in the form of engine characteristic maps. Thus, if the parameter continually determined for the acceleration of the internal combustion engine falls below this threshold value, it is interpreted as the detection of a misfire in the cylinder in question.
A basic problem with these detection methods is that it is very difficult in specific operating phases of the internal combustion engine to differentiate speed variations caused by misfires from operation-related speed variations. Operating phases occurring at high speed and low load are particularly affected. At high speeds the time intervals (segment times) to be measured become shorter and shorter, with the result that it is not possible to define a threshold value that has a sufficiently large gap with respect to the continually determined speed-dependent parameter to allow error-free detection of misfires.
This also applies to operation of the internal combustion engine with non-optimal operating parameters, as is necessary for example for heating catalytic converters. In order to accelerate the heating process, the internal combustion engine is operated for example with an increased quantity of air and fuel, but with very late firing. Consequently, the firing and combustion of the fuel partially take place directly in the catalytic converter and not in the cylinder. The result is a very rapid increase in the exhaust temperature. As the internal combustion engine is operated in this case at a very late firing angle rather than at its optimal firing angle, there is also an increase in the uneven running of the internal combustion engine. This then leads to an increase in and a correspondingly large variance in the acceleration-dependent parameter, which makes misfire detection correspondingly more difficult.
In the prior art numerous algorithms were developed in order to take interfering influences in misfire detection into account and also permit reliable misfire detection even under unfavorable operating conditions of the internal combustion engine. Thus, for example, a switch can be made from one threshold value to another when switching between certain operating phases (catalytic converter heating or not). Numerous further refinements of the algorithms for misfire detection are also known, by means of which there has been a large measure of success in detecting misfires in relatively wide operating ranges of the internal combustion engine with sufficient reliability. However, this must generally be accomplished at the expense of a relatively computing and storage overhead in the operation control device of the internal combustion engine.